The BUB is based on FT232 chip, which is intended only as a USB to serial link. To build a ICSP you need a couple more signals and enough digital logic to coordinate them, this implies a dedicated microcontroller. The FT232 chip does not have accessible programmable logic on the chip so it won't work.

So the BUB is only useful for programming Arduino boards that ALREADY have a bootloader on them, but DO NOT have usb-to serial chips. Early Arduinos used the FT232 but then the Arduino guys started using an Atmel chip which is presumably cheaper. The Atmel chip also has the unfortunate tendency for it's firmware to become corrupt and stop working - necessitating a reflash. In any case, most Arduino type boards already have USB to serial communication, which is the BUB's function.

The BUB is also useful for logging into boards like Raspberry PI with a serial connection from a laptop terminal, using protols like FTP an Telnet.